The need to purify water as, for example, at home (point-of-use water purification systems) is increasing because of the increased incidence of man-made contaminants, the growth of population world wide, and the concomitant increasing burden on the water resources. As more medical and environmental information becomes available about the ability of contaminants in water to cause illnesses, the need to use pure water becomes increasingly evident.
Typical point-of-use water purification systems comprise a bed made of activated carbon that is used to remove chlorine and organic and other impurities. Most often the activated carbon is in granular form. Advantages of the use of an adsorption bed made of granular activated carbon are that granular activated carbon is inexpensive, granular activated carbon in the bed does not compact under flow, and granular activated carbon has relatively little resistance to the flow of water therethrough. A disadvantage of using a granular activated carbon bed is its slow adsorption rate. In industrial applications, it is recommended to have at least 15 minutes contact time between the fluid and the activated granular carbon adsorbent bed. Another disadvantage of a granular activated carbon bed is that, due to the relatively large size of the granules and the void spaces therebetween, it does not remove smaller particulate contaminants.
Powdered activated carbon is known to adsorb impurities faster than granular activated carbon, but the inclusion of carbon fines provides high flow resistance and leads to undesirable compaction under flow. Attempts to avoid compaction commonly involve the incorporation of a binder. While filtration elements comprising activated carbon particles bound into a rigid structure using a polymeric thermoplastic material will not compact under flow, a disadvantage of these so called "carbon-block" structures is that a part of the adsorption capacity of activated carbon is lost due to contact with the immobilizing binding material. Another disadvantage is that, in such filters, the activated carbon occupies only a portion of the adsorption bed volume. The rest of the adsorption bed volume is taken by the immobilizing polymeric binding material. The binder is not active in adsorbing impurities, and its incorporation leads to increased bed sizes as compared to an adsorption bed that contains only activated carbon. Yet another disadvantage of carbon-block material is that its ability to be regenerated or sterilized with steam is hindered due to the presence of the thermoplastic binding material. In addition, making carbon-block material is costly, requiring exact temperatures and other precisely controlled processing conditions.
Activated carbon fiber has been used, but it is expensive and it does compact under flow, leading to moderate flow resistance, although compaction of activated carbon fibers is considerably less than that experienced when employing powdered activated carbon.